1. Field of the Invention
The present general inventive concept relates to a heating roller to fix a toner image, and more particularly, to an apparatus and a method of controlling power supply to a heating roller in which external source power is supplied to a heating resistor included in the heating roller and a phase control circuit usable with the apparatus and method.
2. Description of the Related Art
In a printing device, such as a printer or a copy machine, which forms an image of print data on a printing medium by using a developing material such as toner, a toner image corresponding to the print data is fixed onto the printing medium, and the printing medium is then discharged out of the printing device, thereby obtaining the image of the print data.
The printing device may use a heating roller having heating resistors.
To perform a fixing operation, a surface temperature of the heating roller has to be maintained around a fixing target temperature, for example, 180° C.
The printing device is switched to a print mode when the printing device receives a first printing order after power turns on, or when the printing apparatus receives a printing order while in a stand-by mode.
A time interval between when the printing order is received and before a first printed matter is discharged is referred to as a first print out time (FPOT). In order to reduce the FPOT of the printing apparatus including the heating roller, the surface temperature of the heating roller has to rapidly reach a fixing target temperature.
FIGS. 1A and 1B are diagrams of waveforms illustrating a conventional method of controlling power supplied to a conventional heating roller. If a resistance of a heating resistor is determined in proportion to a temperature of the heating roller while the temperature is equal to or lower than a critical temperature, and a voltage (Vin) 110 shown in FIG. 1A is applied to the heating resistor, then a current (Ir) 120 shown in FIG. 1B flows through the heating resistor.
If the current (Ir) 120 is gradually decreased until the temperature of the heating roller reaches the critical temperature, the conventional method of controlling power supplied to the heating roller has a drawback in that a circuit may be damaged due to excessive current which may flow through the heating resistor when power is initially supplied to the heating resistor. In addition, as a result of a high current flowing through the heating roller in the form of an alternating current, a flicker characteristic is reduced. The flicker characteristic is defined as a phenomenon where power supplied to adjacent circuits is temporarily weakened.
A critical resistance that represents a resistance of a heating resistor at a critical temperature is determined intrinsically. Here, the lower the critical resistance of a used heating resistor is, the more power can be supplied to the heating resistor. Thus, the surface temperature of the heating rollers can be rapidly increased. However, when a heating resistor having a lower critical resistance is used, a higher current will flow through the heating resistor when power is initially supplied to the heating resistor, thereby causing the problems described above. Accordingly, in the conventional method of controlling power supply to a heating roller, a heating resistor having a low critical resistance, that is, a level of resistance not low enough to maximize the power supply to the heating resistor is used, and thus, there is a limitation in reducing the time required for increasing a surface temperature of the heating roller up to a fixing target temperature.
Furthermore, if the printing device receives a printing order immediately after the printing apparatus turns on, the heating roller can be heated only after the printing apparatus, more specifically, a control unit (not illustrated) which controls overall tasks performed in the printing apparatus, for example, a central processing unit (CPU) of the printing apparatus, is initialized. Therefore, the aforementioned problem of having a limitation in reducing a warm-up time during a printing preparation becomes more apparent when the printing apparatus receives the printing order before the initialization of the control unit (not illustrated) is completed.